Presentation Title

Timing Studies for CMS ECAL Calibration Using Z Boson Decays to Electron-Positron Pairs

Faculty Mentor

Maria Spiropulu

Start Date

18-11-2017 10:00 AM

End Date

18-11-2017 10:15 AM

Location

9-239

Session

Physical Sciences 4

Type of Presentation

Oral Talk

Subject Area

physical_mathematical_sciences

Abstract

The Compact Muon Solenoid (CMS) experiment records data from proton-proton collisions at the Large Hadron Collider (LHC) to measure properties of, and probe physics beyond, the Standard Model. The CMS electromagnetic calorimeter (ECAL) is a cylindrical layer of lead tungstate crystals that detect the energies of electrons and photons. To best utilize the ECAL timing capabilities to find new physics, the clocks in every crystal must be synchronized. Z boson decays to electron-positron pairs are used to ascertain and to eventually improve the current ECAL time resolution. Z boson decay data from 2016 yields an ECAL time resolution of 420 picoseconds, much worse than the LHC Run 1 ECAL time resolution of 70 ps. Even after applying two crystal transparency corrections for time resolution, it did not noticeably improve. Investigating the time-of-flight of a single electron/positron through multiple crystals, a time resolution of 140 ps was achieved. In addition to plots of time resolution against ECAL geometry, this local time resolution suggests that the algorithm for combining crystal timestamps on a global level can be refined.

Summary of research results to be presented

In analyzing Z boson decay data in the CERN CMS experiment, we concluded that the LHC (Large Hadron Collider) Run 2 electromagnetic calorimeter (ECAL) time resolution has noticeably degraded compared to the LHC Run 1. We have identified three sources of this degradation. In decreasing significance: the increased luminosity and pileup, the systematic calibration of the lead tungstate crystal geometries, and the correction for worsening crystal transparencies are all contributing to the worse time resolution. Additionally, these effects were found to be negligible at the local crystal level, and only appear in higher-level processes for global crystal synchronization. The implication of this is that the ECAL time resolution can be dramatically improved to near LHC Run 1 levels. With this improved resolution, the ECAL timing properties can be used to search for supersymmetric theories (e.g. displaced and delayed hadronization jets), and other physics beyond the standard model.

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Nov 18th, 10:00 AM Nov 18th, 10:15 AM

Timing Studies for CMS ECAL Calibration Using Z Boson Decays to Electron-Positron Pairs

9-239

The Compact Muon Solenoid (CMS) experiment records data from proton-proton collisions at the Large Hadron Collider (LHC) to measure properties of, and probe physics beyond, the Standard Model. The CMS electromagnetic calorimeter (ECAL) is a cylindrical layer of lead tungstate crystals that detect the energies of electrons and photons. To best utilize the ECAL timing capabilities to find new physics, the clocks in every crystal must be synchronized. Z boson decays to electron-positron pairs are used to ascertain and to eventually improve the current ECAL time resolution. Z boson decay data from 2016 yields an ECAL time resolution of 420 picoseconds, much worse than the LHC Run 1 ECAL time resolution of 70 ps. Even after applying two crystal transparency corrections for time resolution, it did not noticeably improve. Investigating the time-of-flight of a single electron/positron through multiple crystals, a time resolution of 140 ps was achieved. In addition to plots of time resolution against ECAL geometry, this local time resolution suggests that the algorithm for combining crystal timestamps on a global level can be refined.